This invention relates to hybrid feedback amplifier of a push-pull type. The amplifier is specifically adapted for use in a line repeater for a carrier telephone transmission system, such as a 12-MHz or a 60-MHz coaxial transmission system, or in a repeater for a CATV system.
The important requisites to such an amplifier are (1) a small distortion factor for avoiding nonliner cross talk, (2) a sufficient gain for precisely and stably equalizing the transmission line loss, and (3) an input and/or output impedance matched to the line impedance for obviating reflection of signals. The distortion factor mentioned above as the requisite (1) may be reduced by negative feedback techniques and by adoption of a push-pull construction which cancels the second-order distortion dominant in the distorted components. The negative feedback techniques in general are described by Hendrik W. Bode in a book entitled "Network Analysis and Feedback Amplifier Design" and published in 1954 by D. van Nostrand Company, Inc. It is also stated in the referenced book, at pages 35- 39, that hybrid feedback is excellent for simultaneously satisfying the above-mentioned requisites (2) and (3).
It is usual on resorting to the hybrid feedback techniques to use a transformer as a hybrid coil. This, however, is in contradiction to rendering an amplifier broad-banded particularly at higher frequencies. In order to overcome the defect, it has been proposed in U.S. Pat. Nos. 3,487,325 and 3,820,036 to use an autotransformer as the hybrid coil. In order to make a push-pull amplifier fully achieve the cancellation of distortion, it is a well-known important requisite to make the circuitry completely symetric so as to avoid unbalance in the stray capacitance, the leakage inductance, and the like. While attempts have been made to provide a wide-band high-frequency amplifier of a small distortion factor by combination of known techniques, mere adoption of the hybrid feedback to a push-pull amplifier inevitably results in complicated circuitry.